Sonex + Rotax = Sontax?

[ViennaVA223]

Greetings from scenic (okay, dark and rainy) Vienna, Virginia, too close to the Washington Beltway!

I haven't bought a Sonex kit yet (still working on getting the house ready to sell and moving to less expensive Indiana), but I did go to the June 2014 Sonex workshop (Jeremy Monnett taught me how to buck rivets with the two-pound hammer) and fly for a couple of hours with Joe Norris.

I love the Sonex airframe (despite the tight shoulder squeeze with Joe), but am still wondering about the engine choices. Frankly, I found the AeroVee performance rather anemic, and OSH is only about 1,000 feet MSL. The Jabiru 3300 was more to my taste; maybe it's all those years towing gliders in Super Cubs and Pawnees--with no "dope on the rope," the climb rates and angles are pretty healthy in both towplane types.

Meanwhile, I've been reading a lot about the Rotax engines, and I'm particularly impressed with the fuel economy attained by the 914iS on premium auto gas via direct fuel injection. I'm also greatly interested in seeing the Rotax 915iS, scheduled for first deliveries in late 2017, with 135 hp as the design goal. The 915iS uses the same block as the 912 variants, I think, but adds both direct fuel injection AND turbocharging. As I would like to leap the Rocky Mountains from time to time, that looks pretty exciting.

Yes, I know the Rotax engines are complicated and more expensive than an AeroVee. And I'm not an engines guy. Just LOOKING at a photo of a Rotax gives me the willies. However, partly to overcome that, and to satisfy my curiosity, I'm going to attend three Rotax maintenance schools at Lockwood Aviation next month.

I happened to notice that the all-up weight projected for the Rotax 915iS is 185.2 pounds. The weight of the AeroVee Turbo is 185 pounds, total, according to the Sonex website. So more dollars and more complexity and sophistication on the Rotax side add up to 35 more horsepower--that's 35 PERCENT more horsepower, too!--than the AeroVee Turbo, for the same weight. With direct fuel injection, the Rotax is likely going to deliver substantially better specific fuel consumption than the AeroVee Turbo.

So that's the direction my current fantasies are headed: Sonex airframe + Rotax (912is? 914? 915iS?) = Sontax. And now you know my innermost, darkest secrets! LOL

Anyone else beginning to think along these lines? I hope I've stimulated a discussion, an argument, a flaming, an intergalactic food fight.

Tailwinds,

Jan W. Steenblik (CML/ASEL/ASES, IFR airplane, private glider)
Vienna, VA

[kmacht]

By the time you build a sonex with an expensive Rotax you could build an RV 4 or 7 for about the same price. The airframe cost is about the same and if going Rotax you can get a a small Lycoming for the same price. If you already don't care for the shoulder room in the Sonex and don't like the engine why not just build something else that is already designed to meet your needs. Not being an engine guy typically doesnt go well with trying to develop an new unsupported engine package. You need much more than just an engine. Think custom cowl, custom engine mount, custom nose wheel if not going tail dragger, different fuel setup, intake, exhaust, baffling, etc.. I love my sonex for what it is but that doesn't mean it fits every mission profile. If it doesn't fit yours look around to see what else might. There are thousands of different airplanes out there.

Keith

[sonex892.]

I think a rotax in a sonex would be a great match, and a rotax with 135hp wow.

The weight is not a problem with a Rotax. It would be no more an engineering challenge than fitting a corvair or viking. Its not rocket science. There are now quite a few sonex's operating with rotax engines.

Experimenting is what experimental aviation is about.

Steve
Sonex 892
3300 jab 180hrs

[ViennaVA223]

Keith--

The main reason I won't be building or buying an RV-4, RV-7, or any other non-LSA "real airplane" is that, since 2009, I've been taking an antidepressant, duloxetine (Cymbalta), that is not one of the four antidepressants that the FAA approves for use by pilots and air traffic controllers.

I could try to convince the agency that I have absolutely no side effects from the medication and have never suffered "suicidal ideation," apparently one of their concerns, but the risk is too great: If I'm DENIED a medical certificate, I lose the right to fly an LSA. That's not worth it to me.

Also, I recall looking at the Van's Aircraft website a while back. The site said that the company is able to buy Lycoming engines in bulk, so they offer very competitive prices. Well, even the 160-hp four-banger (O-320) listed for about $30k, and the prices for Lycomings went up from there, to about $50k. The O-233 that Lycoming developed for the LSA market probably is somewhat less expensive, but I think Rotax really is the technology leader right now.

And I did say that I like the Sonex airframe; it's a good design, although it's a compromise, as all aircraft are.

My interest in hanging a Rotax on a Sonex would not be to increase cruise speed, but climb performance. I'm not an aeronautical engineer, but my fuzzy recollection is that, after you provide enough ponies to sustain level flight, if you add more horsepower, you can convert that to climb, and 135 hp--with a critical altitude way above what even a Rockies-hopping LSA would be allowed (I'm looking for 13,500' MSL to cross Monarch Pass in Colorado)--would put both the AeroVee Turbo and the Jabiru 3300 to shame.

Steve: Thanks for the support!

Tailwinds,
Jan

[mike20sm]

I'm interested in the UL power engine line and the thing brought up recently was the propeller size needed for whatever alternative engine's sweet spot. It's an important thing to consider pertaining to the ground clearance and how long a prop you can swing.

[Bryan Cotton]

Jan,
I think a Rotax in a Sonex would be cool, but I would not do it myself. I would build an RV first, but am not as concerned about the medical. If you really like the Rotax, and think the Sonex is tight, an RV12 is worthy of consideration. It is a great kit from a great company, and is an LSA. I voted with my wallet for Sonex, because the value per dollar is excellent and I can do a little acro in it. But if size and a Rotax engine are important to your plans the RV12 comes out ahead, though at a price.

[ViennaVA223]

Mike20sm:

Thanks for the reminder! Yes, prop clearance would be a VERY important consideration. Can anyone out there advise on what the prop clearance is in level position with tailwheel Sonex (i.e., as in wheel landing) with current props?

Maybe this Rotax notion of mine would require a three- or four-bladed prop to keep the prop within a reasonable diameter and still convert the horsepower to thrust properly. That, too, could be done. Maybe I'll ask Lonnie Prince what he thinks.

Anyone have any thoughts on that aspect?

Jan

[JT1974]

There are several Rotax installations on a Sonex. Search YouTube for "Sonex Rotax" and you'll find them.

[sonex892.]

Thanks for the reminder! Yes, prop clearance would be a VERY important consideration. Can anyone out there advise on what the prop clearance is in level position with tailwheel Sonex (i.e., as in wheel landing) with current props? Jan

With non standard 5:00x5 tires and my one off 57" dia prop, I measure 10.5" prop clearance with the fuselage level. The larger tires make it maybe an inch taller than with the standard tires.

Steve
Sonex 892

[peter anson]

I used to run a 60" prop, so 3" less clearance with the normal small tyres and it wasn't a problem because you land in a tail low attitude. Had to be a bit more careful ground handling. I think most of the builders who have fitted Rotaxes extend the landing gear a bit.

Peter Anson
Sonex 894